In a cellular network, such as one employing orthogonal frequency division multiplexing (OFDM) or orthogonal frequency division multiple access (OFDMA), each cell employs a base station that communicates with user equipment, such as a cell phone, a laptop, or a PDA, that is actively located within its cell. When the user equipment is first turned on, it has to do an initial cell search in order to be connected to the cellular network. This involves a downlink synchronization process between the base station and the user equipment wherein the base station sends a synchronization signal to the user equipment.
During initial cell search, the user equipment establishes timing and frequency offset parameters. Timing involves knowing where to sample the start of the synchronization frame and associated symbols. Frequency offset involves determining the mismatch between the controlling oscillator at the base station and the local oscillator in the user equipment.
Depending on the quality of the local oscillator, the frequency offset may be large and require considerable search time as well as additional algorithms to accommodate. This effect is exacerbated if the user equipment is moving at car or train speeds. In addition to timing and frequency considerations, some information that is specific to the initial cell, such as Cell-ID, has to be acquired. Since downlink synchronization involves several operations, the design and procedure of downlink synchronization shall attempt to minimize the receiver complexity and time required for cell search.
As the moving user equipment approaches a cell boundary between two adjoining cells, it performs a neighboring cell search in preparation to handover its activation from the initial cell to the neighboring cell. During this time, it receives information from two or more base stations. Since the base stations typically employ a common synchronization code, this common signal causes a mismatch between the channel experienced by the data transmission and the transmitted synchronization signal for the user equipment thereby resulting in a reduced performance. This phenomenon occurs especially in a tightly synchronized network, such as those deployed in the USA, and has become increasingly popular with medium to large cell radius. While this phenomenon is also relevant to the initial cell search, it is particularly problematic for the neighboring cell search as the operating signal-to-noise ratio (SNR) for the neighboring cell search is considerably lower. This performance reduction translates to larger cell search time, which may result in higher disconnect probability upon handover.
Accordingly, what is needed in the art is an enhanced way to accomplish initial and neighboring cell searches.